(1) Field of the Invention
The present invention relates to an automatic transmission system provided with a function of engine braking, particularly to an improvement thereof, in which the ranges of the vehicle speed can be changed from one to another under an electronic control or an hydraulic control.
(2) Description of the Prior Art
In a hydraulically controlled conventional automatic transmission system, when a manual valve thereof is maintained in a range of automatic speed change, that is, in a "D"-range in the case of down slope running, an engine braking function can hardly be expected. Thus, particularly in the case of steep and long down slope running, it is required each time for a driver to operate the manual valve manually from the D-range to a second fixed speed range or to a first fixed speed range, in order to effect the engine braking without the frequent necessity of operating the foot brake.
Regarding the above, the matter is the same in a conventional electronically controlled automatic transmission system. For the purpose of preventing the above troublesome matter, an electronically controlled or hydraulically controlled automatic transmission system has been developed such that, when an accelerator pedal is released and a brake pedal is depressed, or stepped upon, the range of vehicle speed can be changed automatically from the D-range to some other preceding lower range regardless of the maintenance of the setting of the control means for the D-range.
However, in the conventional automatic transmission system of the above kind, whenever the brake pedal is stepped upon for lowering the vehicle speed, the above mentioned down shift of vehicle speed range is effected even in the case of running on a flat road in which the engine braking is not required at all, so that an uncomfortable feeling caused by the lowered speed is impressed to passengers each time of braking, and, as a result, the feel of the driving is impaired.